Question: Solve for $x$ and $y$ using substitution. ${-x-5y = 7}$ ${y = -x-3}$
Answer: Since $y$ has already been solved for, substitute $-x-3$ for $y$ in the first equation. ${-x - 5}{(-x-3)}{= 7}$ Simplify and solve for $x$ $-x+5x + 15 = 7$ $4x+15 = 7$ $4x+15{-15} = 7{-15}$ $4x = -8$ $\dfrac{4x}{{4}} = \dfrac{-8}{{4}}$ ${x = -2}$ Now that you know ${x = -2}$ , plug it back into $\thinspace {y = -x-3}\thinspace$ to find $y$ ${y = -}{(-2)}{ - 3}$ $y = 2 - 3$ $y = -1$ You can also plug ${x = -2}$ into $\thinspace {-x-5y = 7}\thinspace$ and get the same answer for $y$ : ${-}{(-2)}{ - 5y = 7}$ ${y = -1}$